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An Isolable and Monomeric Phosphorus Radical That Is Resonance-Stabilized by the Vanadium(IVV) Redox Couple.

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Angewandte
Chemie
DOI: 10.1002/ange.200700059
Phosphorus Radicals
An Isolable and Monomeric Phosphorus Radical That Is ResonanceStabilized by the Vanadium(IV/V) Redox Couple**
Paresh Agarwal, Nicholas A. Piro, Karsten Meyer, Peter Mller, and Christopher C. Cummins*
Dedicated to Professor Herbert W. Roesky
The synthesis of isolable radicals involving the heavier maingroup elements represents a significant chemical challenge.[1]
Sterically stabilized phosphorus-centered radicals which are
persistent and monomeric in solution have been synthesized.[2–4] Of particular note are the “jack-in-the-box” diphosphines of Lappert, Power, and co-workers, which dissociate
into sterically stabilized and persistent radicals upon solvation.[5, 6] Also, a p-phosphaquinone radical anion has been
studied[7] and the resonance-stabilized 1,3-diphosphaallyl[8]
and diphosphanyl[9] radicals have been isolated. Yet the
synthesis of a neutral phosphorus radical that exists as a
monomer even in the solid state has been elusive. Herein we
report such a phosphorus radical, a radical that is electronically stabilized by the ability to delocalize its P-radical
character onto a pair of nitridovanadium metalloligands.
Prior to tackling the synthesis, our quantum chemical
calculations predicted that the magnitude of the electronic
stabilization relative to CPH2 would be on the order of
24 kcal mol1.
One significant difference between transition metals and
the heavier main-group elements is that the former are
generally susceptible to one-electron redox chemistry,
whereas the latter are more limited to electron-pair chemistry.[1] This distinction prompted our choice of two nitridovanadium(V) trisanilide moieties as ligands for the stabilization
of divalent phosphorus. Hence, contributing resonance structures with vanadium(IV) character would stabilize a Pcentered radical (Scheme 1). In addition, N-neopentylanilide
ligands would add a degree of steric protection to a radical
with the target formula [CP{NV[N(Np)Ar]3}2] (1; Np = neopentyl, Ar = 3,5-Me2C6H3).
The reaction of sodium azide with the vanadium(III)
precursor [V{N(Np)Ar}3] (2) cleanly generates the sodium
salt of nitride anion [NV{N(Np)Ar}3]1 (3) as a fine yellow
Scheme 1. Resonance stabilization of radical 1 provided by the vanadium(IV/V) redox couple.
[*] P. Agarwal, N. A. Piro, Dr. P. M0ller, Prof. Dr. C. C. Cummins
Department of Chemistry
Massachusetts Institute of Technology, Room 6-435
Cambridge, MA 02139-4307 (USA)
Fax: (+ 1) 617-258-5700
E-mail: ccummins@mit.edu
Prof. Dr. K. Meyer
Friedrich-Alexander-Universit>t Erlangen-N0rnberg
Institute of Inorganic Chemistry
Egerlandstrasse 1, 91058 Erlangen (Germany)
[**] We thank the U.S. National Science Foundation for support of this
research through grant CHE-0316823 and through a predoctoral
fellwoship to N.A.P, and the MIT Undergraduate Research Opportunities Program for support of P.A. We also would like to thank
Prof. Dr. Anthony Spek (Utrecht) for his validation of the crystal
structure of 1.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. 2007, 119, 3171 –3174
powder in a single step in 77 % yield. A similar reaction has
been described previously for the tert-butyl derivative [V{N(tBu)Ar}3].[10] We exploited the nucleophilicity of anion 3
through its reaction with 0.5 equivalents of PCl3 to generate
the red-brown radical precursor [ClP{NV[N(Np)Ar]3}2] (1Cl) in 48 % yield after separation from [ClV{N(Np)Ar}3] (2Cl), which is generated as a by-product of the reaction, as
confirmed by independent synthesis.[11] Each pair of neopentyl methylene protons on the anilide ligands of 1-Cl is
diastereotopic as assessed by 1H NMR spectroscopy, owing to
the pyramidal geometry at the phosphorus atom.
Access to target radical 1 is provided by the one-electron
reduction of 1-Cl, which can be effected by a variety of
reducing agents. One such reductant, the potent chlorineatom abstractor [Ti{N(tBu)Ar}3] ,[12–14] reacts rapidly with 1-Cl
in diethyl ether solution to smoothly generate [ClTi{N-
2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Zuschriften
(tBu)Ar}3] and the radical 1, the identities of which were
confirmed by NMR and EPR spectroscopy (see below). This
reaction suggests that the chlorine atom in 1-Cl is sufficiently
sterically accessible to form a bridge to the crowded titanium
center in the process of Cl-atom transfer. For synthetic
purposes and for easy separation, 1-Cl can also be cleanly
reduced to 1 with potassium graphite.[3] Crystallization of the
reaction product from pentane provides dark brown crystals
of 1 in 77 % yield. The 1H NMR spectrum of 1 consists only of
two broad resonances at d = 1.4 and 2.8 ppm. The solution
magnetic susceptibility of 1 was measured by the Evans
method[15, 16] and yielded a magnetic moment of 1.67 mB,
consistent with the presence of a single unpaired electron.
The EPR spectrum of 1 shows a complex splitting pattern.
This spectrum was modeled with hyperfine couplings to two
51
V nuclei (I = 7/2, 99.75 %, Aiso = 23.8 G) and one 31P nucleus
(I = 1/2, 100 %, Aiso = 42.5 G).[17] The simulated and experimental spectra are presented in Figure 1. The large value of
Table 1: Hyperfine coupling constants and g values of selected PII and VIV
radicals in toluene at about 25 8C (unless otherwise indicated).
Radical species
[P{NV[N(Np)Ar]3}2]
P(OMes*)2
P(Mes*)(OtBu)
P[CH(SiMe3)2]2
P[N(SiMe3)2]2
P[Mes*][C6H2(tBu)2O]
P[CH(SiMe3)2][NiPr2]
P[NiPr2][N(SiMe3)2]
P[CH(SiMe3)2][NMe2]
[ClV{N(Np)Ar}3]
[ClV{(Me3SiNCH2CH2)3N}]
[V(CH2SiMe3)4]
[V(NMe2)4]
[V(NEt2)4]
Aiso [G]
gav
31
51
42.5 ( P), 23.8 ( V)
82 (31P)
100 (31P)
96.3 (31P)
91.8 (31P)
93 (31P)
63 (31P), 3.7 (14N)
77.2 (31P), 5.2 (14N)
65 (31P)
67.9 (51V)
147 (51V)
54.5 (51V)
65 (51V)
66 (51V)
1.984[a]
1.999[19]
2.005[19]
2.009[20]
2.008[20]
2.007[b],[4]
2.005[20]
2.007[20]
2.008[20]
1.973[a]
1.963[c],[21]
1.968[22]
1.975[d],[23]
1.977[e],[24]
[a] This work. [b] THF (295 K). [c] Toluene glass (92 K). [d] Frozen CyMe
(120 K). [e] Benzene (ca. 300 K). Mes* = 2,4,6-tBu3C6H2.
Figure 2. SOMO of 1 m, showing delocalization of the radical throughout the V-NPN-V p system.
Figure 1. Observed and simulated EPR spectrum of 1. Hyperfine
coupling constants and g values are listed in Table 1.
the 31P coupling is indicative of substantial radical character
on the phosphorus atom, while the 51V coupling indicates
some degree of vanadium(IV) character. A comparison
of these values to those of other phosphorus(II) and
vanadium(IV) complexes can be found in Table 1. These
data show that both the 51V and 31P hyperfine couplings are
smaller than in traditional localized PII and VIV radicals,
consistent with a significant degree of delocalization.
To further illuminate the delocalization of the unpaired
electron in this system, quantum chemical calculations on the
model system [CP{NV[N(Me)Ph]3}2] (1 m) were performed by
using the ADF package,[18] with 1 m restricted to
C2 symmetry; the rotational axis was the z axis and the
NPN moeity was positioned in the xz plane. These calculations predict that the major orbital contributions to the
SOMO (Figure 2) are the phosphorus 3py orbital (31.30 %)
and the vanadium 3dxy (39.49 % over two vanadium atoms)
and 3dx2y2 (8.33 %) orbitals. The combined EPR and computational data indicate that both the phosphorus-centered and
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www.angewandte.de
vanadium-centered radical resonance structures (Scheme 1)
contribute substantially to the electronic structure of 1.
The solid-state structure of radical 1 was determined by
single-crystal X-ray diffraction,[25] which revealed a monomer
with no close contacts involving the two-coordinate phosphorus atom. The structure shows the expected geometry with a
bent N-P-N linkage (110.9(3)8; Figure 3 a). Viewed from
another perspective, radical 1 can be thought of as a mixedvalence vanadium(IV/V) system with a bridging NPN ligand,
a ligand which to our knowledge has not been described
previously in transition-metal chemistry. This ligand is
formally a heavy-azide analogue, a class of molecules that
has garnered attention from our group and others.[26–30] The
electron delocalization represented by the resonance structures in Scheme 1 has the effect of contracting the PN bonds
by about 0.055 I and lengthening the VN bonds by
approximately 0.036 I when compared to localized single
PN and double VN bonds, such as those in 1-SePh (see
below).
Radical 1 undergoes reactions at its phosphorus center to
generate new phosphorus–element bonds. For example,
reaction between 1 and 0.5 equivalents PhEEPh (E = S, Se)
cleanly
generates
dark
red-brown,
diamagnetic
[PhEP{NV[N(Np)Ar]3}2] (1-EPh). That bond formation
takes place at the phosphorus center was confirmed by the
2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. 2007, 119, 3171 –3174
Angewandte
Chemie
using DFT methods. The PH bond in 1 m-H was computed
to be 24 kcal mol1 weaker than that in PH3 (using the
isodesmic reaction 1 m-H + PH2 !1 m + PH3, for which DE =
24 kcal mol1), thus putting the PH bond strength in 1-H at
roughly 58 kcal mol1.[33] The predicted weakness of the PH
bond in 1 m-H is indicative of a large stabilization provided by
the delocalization of the unpaired electron as discussed
above. Accordingly, the synthesis of 1-H, a potential potent
H-atom donor, has proven to be elusive. No reaction was
observed between 1 and the common H-atom sources
nBu3SnH, nBu2SnH2, and [(h5-C5H5)(CO)3MoH][34, 35] in
diethyl ether at room temperature. A synthesis of 1-H from
1-Cl was attempted by using one equivalent of LiHBEt3 as a
hydride source. When LiHBEt3 was added to a solution of 1Cl at room temperature, a rapid reaction was observed with
vigorous gas evolution. When assayed by low-temperature
1
H NMR spectroscopy in [D8]toluene, the reaction did not
proceed below 20 8C. As the reaction mixture warmed, the
generation of H2 (d = 4.49 ppm) and 1 was observed. On the
basis of bond-dissociation enthalpy calculations of 1 m-H, the
generation of H2 from two equivalents of the putative
intermediate 1-H is a thermodynamically feasible process
[Eq. (1)].
1-Cl þ LiHBEt3 ! 1 þ
Figure 3. a) Molecular structure of 1 with 50 % probability ellipsoids.
Selected bond lengths [H] and angles [8]: V1-N4 1.724(2), P1-N4
1.630(7), P1-N4a 1.614(7); P1-N4-V1 148.9(3), N4-P1-N4a 110.9(3),
P1-N4a-V1a 150.8(3). b) Molecular structure of 1-SePh with 50 %
probability ellipsoids. Selected bond lengths [H] and angles [8]: C1-Se1
1.910(4), Se1-P1 2.3021(9), P1-N8 1.674(2), P1-N4 1.681(2), V1-N4
1.6824(19), N8-V2 1.6930(19); C1-Se1-P1 101.62(15), N8-P1-N4
103.63(10), N8-P1-Se1 98.23(8), N4-P1-Se1 102.49(8), P1-N4-V1
162.68(13), P1-N8-V2 160.46(14).
presence of only a single anilide ligand environment as
assessed by 1H NMR spectroscopy (again with diastereotopic
neopentyl methylene units), by the presence of 31P,77Se
coupling (JPSe = 327 Hz) in the 77Se NMR spectrum of 1SePh, and by a single-crystal X-ray diffraction study on 1SePh (Figure 3 b). Additionally, treatment of 1 with 0.5 equivalents of p-tetrachlorobenzoquinone generates dark-red,
diamagnetic [{{[Ar(Np)N]3VN}2P}2(m-OC6Cl4O)] (4). The
1
H NMR spectrum of 4 is similar to those of 1-EPh, except
that the resonances, especially those of the methylene and
ortho aryl protons, are broadened in a manner indicative of
restricted rotation. All three of these radical-reaction products are generated in nearly quantitative yield (> 95 %), as
assessed by 1H NMR spectroscopy in the presence of an
internal standard. Treatment of radical 1 with milder reagents
including white phosphorus and GombergJs dimer, both of
which often react with less-stabilized radicals,[3, 31, 32] gave rise
to negligible reaction at room temperature.
As a measure of the radical stability in 1, the homolytic
bond-dissociation energy of the PH bond in the model
compound [HP{NV[N(Me)Ph]3}2] (1 m-H) was calculated by
Angew. Chem. 2007, 119, 3171 –3174
1
H þ LiCl þ BEt3
2 2
ð1Þ
Herein we have reported on [CP{NV[N(Np)Ar]3}2] (1), a
neutral phosphorus radical that is a stable monomer even in
the solid state. While this molecule undergoes selected radical
bond-formation reactions at its phosphorus center, the
resonance stabilization provided by the two vanadium metalloligands tempers its reactivity to a significant extent. It is
expected that the P-radical character of the system may be
tuned, for example, by changing the metal center in the
metalloligands (for example, Nb in place of V). In addition,
the easily synthesized redox-active metalloligand[36, 37]
employed here may be useful for the stabilization of radicals
centered on other heavy main-group elements.
Received: January 5, 2007
Published online: March 12, 2007
.
Keywords: metalloligands · phosphorus · radicals ·
resonance stabilization · vanadium
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Angew. Chem. 2007, 119, 3171 –3174
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